Change Travis to Ubuntu 20.04 LTS (focal)

.travis.yml

@@ -1,4 +1,4 @@
-dist: trusty
+dist: focal
 before_install:
   - sudo apt-get install -y ninja-build
   # Arduino IDE adds a lot of noise caused by network traffic, trying to firewall it off
@@ -32,4 +32,4 @@ script:
   - bash -x build.sh || { echo "1_75mm_MK2-RAMBo13a-E3Dv6full variant failed" && false; }
   - rm Firmware/Configuration_prusa.h
   - cp Firmware/variants/1_75mm_MK2-RAMBo10a-E3Dv6full.h Firmware/Configuration_prusa.h
-  - bash -x build.sh || { echo "1_75mm_MK2-RAMBo10a-E3Dv6full variant failed" && false; }
+  - bash -x build.sh || { echo "1_75mm_MK2-RAMBo10a-E3Dv6full variant failed" && false; }